1. Field of the Invention
The present invention relates to a method of determining at least one parameter representative of at least one interaction along a longitudinal direction between a vehicle tyre and the ground.
In particular, the invention concerns determining of a parameter representative of the longitudinal force to which the tyre is submitted in the footprint area and/or of the tyre slippage in a longitudinal direction.
The invention also relates to an apparatus for determining said parameter.
2. Description of the Related Art
In the present context, the contact region between the tyre and ground will be referred to as “footprint area”.
Presently there is a need for determining the interactions generated between the tyre and ground during running of the tyre itself, in particular at the footprint area.
In fact, evaluation of these interactions is critical for a reliable determination of possible danger situations when the ground conditions are not optimal or the vehicle's speed is too high as compared with the vehicle's travel conditions.
As above said, the interactions that are herein taken into account are those generated in a longitudinal direction, i.e. the direction along which the vehicle to which the tyre is mounted runs.
Determining said interactions enables an indirect evaluation of the friction generated between the tyre and ground, thus allowing the electronic system mounted on the vehicle to automatically act on the vehicle movements, above all in case of sudden accelerating or braking actions, which conditions are generally riskier for the driver who will lose the vehicle's control.
The U.S. Pat. No. 5,913,240 discloses a system for determining the longitudinal forces generated between the tyre and ground, which system comprises at least one pair of markers mounted on the tyre to different radial distances from the hub, and at least one pair of detectors fixedly mounted on the vehicle chassis so as to detect passage of the markers at each revolution of the wheel.
Depending on the phase displacement between the two detection operations, which phase displacement is representative of the deformation to which the tyre has been submitted following braking or acceleration for example, the longitudinal force generated in the footprint area is calculated.
The U.S. Pat. No. 6,550,320 discloses a system for determining the forces generated at the footprint area by exploiting the detection of a SWT (Side Wall Torsion) sensor of the magnetic type and the processing operations carried out through a neural network or bilinear equations. In particular, the following forces acting on the tyre can be calculated: side force, circumferential torque, longitudinal force and vertical force.
The U.S. Pat. No. 6,904,351 discloses a control system for vehicles in which the longitudinal force exerted on the tyre is calculated as a function of the driving torque applied to the wheel, the braking torque applied to the wheel, the vertical force, the moment of inertia of the wheel, the rotation speed of the wheel and the radius of the wheel itself.
The Applicant has found that the systems of known type such as those briefly described above, have a significant complexity and therefore require hardware-software resources of high quality to obtain reliable measurements.
In particular, the Applicant has observed that a system of the type described in U.S. Pat. No. 5,913,240 draws the evaluation of the longitudinal force from detection of the position of two points that are radially very close to each other, i.e. the position of the two markers located on the tyre sidewall; said detection must be therefore very accurate, to ensure reliability of the subsequent calculation of the forces.
The Applicant has also noticed that for systems of the type described in U.S. Pat. No. 6,550,320 and U.S. Pat. No. 6,904,351, a high calculation capacity through which the longitudinal force is calculated, is required, due to the high number of variables on which the intensity of the different magnitudes to be determined depends.
As above said, in the present context by “footprint area” it is intended the portion of outer surface of the tyre in contact with the ground at a given time instant.
More specifically, the footprint area is herein defined between a first longitudinal end corresponding to the point at which the tyre comes into contact with the ground, and a second longitudinal end corresponding to the point at which the tyre separates from the ground.
In the above defined context the Applicant in particular has felt the necessity to be able to determine the interactions between tyre and ground, and specifically those generated along the longitudinal direction, in a simple manner and therefore according to algorithms that do not involve an excessive calculation complexity.